While I kind of like Team Fortress 2 slightly better Battlefield Heroes is still worth a play.
It's in the same style of multiplayer gameplay as the Battlefield series where you play different types of multiplayer modes such as capture the flag or team death-match on an open map where you get instant access to vehicles such as jeeps, tanks, and planes. This type of style of multiplayer to me feels more liberating than other multiplayer games where you have wait until you get an odd number of kills in order to have access to a chopper.
The style of this game is cartoonish with a World War 2 theme, which is very humorous. I kind of like the fact that unlike Team Fortress 2 you can customize your characters looks and appearances right from the start. However I do feel that some of the character models like very lifeless compared to Team Fortress 2, which had very expressive and unique faces. Another thing I didn't really like is the fact that there are only 3 classes in Battlefield Heroes with only a few useful ones where as Team Fortress 2 has 9 classes with each being very useful.
As you play through the game you'll eventually earn some credit that allows you to unlock new abilities like 3-grenade toss, poison knife, flaming bullets and tons more. The only downside is that when playing on a keyboard the abilities are on the number keys and it can be very difficult to select the abilities while you're in the middle of combat.
Whether you prefer Battlefield Heroes or Team Fortress 2 is entirely up to your personal preferences. Both of these are very fun free to play multiplayer games out there right now. While I kind of like Team Fortress 2 slightly better Battlefield Heroes is still worth a play.